1. Field of the Invention
This invention relates to a laser exposing apparatus for use in an image forming apparatus such as a copying machine or a printer using an electrophotographic process.
2. Description of the Related Art
Apparatuses for effecting image exposure by a laser beam modulated in conformity with image information has spread.
In such an apparatus, the higher speed of the apparatus can be achieved by exposing a photosensitive member by a plurality of laser beams.
FIG. 1 of the accompanying drawings shows a conventional construction. FIG. 1 is a plan view, and FIG. 2 of the accompanying drawings is a side view. In FIGS. 1 and 2, the reference numeral 15 designates a rotary polygon mirror, and the reference numeral 16 denotes a laser scanner motor for rotatively driving the rotary polygon mirror 15. As the rotary polygon mirror 15, use is made of a hexagonal one. The reference numeral 17 designates a laser diode which is a light source for recording. The laser diode 17 is a two-beam laser diode generating two laser beams from a stem. The laser diode 17 is turned or turned off in conformity with an image signal by a laser driver, not shown, and the light-modulated laser beams emitted from the laser diode 17 are applied toward the rotary polygon mirror 15 through a collimator lens 20. The rotary polygon mirror 15 is rotated in the direction of arrow, and the laser beams emitted from the laser diode 17 are reflected as deflected beams continuously changed in angle with the rotation of the rotary polygon mirror 15 by the reflecting surface thereof. These reflected lights are subjected to the correction of distortion aberration by an f-θ lens 21, and scan in the main scanning direction of a photosensitive drum 10 via a reflecting mirror 18. At this time, one beam corresponds to the scanning of one line, and by the scanning of one surface of the rotary polygon mirror 15, two lines scan in the main scanning direction of the photosensitive drum 10.
The photosensitive drum 10 is electrified in advance by an electrifier 11, and is sequentially exposed by the scanning of the laser beams, whereby an electrostatic latent image is formed. Also, the photosensitive drum 10 is rotated in the direction of arrow, and the electrostatic latent image formed thereon is developed by a developing device 12, and the developed visible image is transferred to a transferring sheet, not shown, by a transferring electrifier 13. The transferring sheet to which the visible image has been transferred is conveyed to a fixing device 14, where fixing is effected, whereafter the transferring sheet is discharged out of the image forming apparatus.
Also, a BD sensor 19 is disposed at a location near or corresponding to a scanning start position in the main scanning direction on the side of the photosensitive drum 10. The laser beam reflected by each reflecting surface of the rotary polygon mirror 15 is detected by the BD sensor 19 prior to the scanning of the line. The detected BD signal is inputted to a timing controller, not shown, as a scanning start reference signal in the main scanning direction, and with this signal as the reference, the synchronization of a writing start position of each time in the main scanning direction is taken.
In the above-described example of the prior art, however, the distance from the light emitting surface of the laser to the photosensitive member has differed between the two laser beams, and an evil thereby has occurred. An optical path at this time is shown in FIG. 3 of the accompanying drawings. In FIG. 3, the same reference numerals as those in FIG. 1 designate the same members. In FIG. 3, the letters A and B designate the centers of the two beams, respectively. Usually, in an electrophotographic apparatus, in order to prevent a laser beam applied to a photosensitive member from being reflected, returning reversely along an optical path, and being again applied to a laser diode chip to thereby destroy the chip, use is made of oblique incidence in which the laser beam is obliquely applied to the surface of the photosensitive member as shown so that the reflected light may not return. As the result, however, in the case of a system in which a plurality of lines are scanned at a time by a plurality of laser beams, the lengths of optical paths have become different therebetween. In this example, the optical path B is longer than the optical path A.
The evil at this time is shown in FIG. 4 of the accompanying drawings. In FIG. 4, the same reference numerals as those in FIG. 1 designate the same members. In FIG. 4, A and B show that images scanned from the polygon mirror 15 to the photosensitive member 10 through the f-θ lens 21 differ in magnification from each other, and the image scanned by the beam A is smaller in magnification than the image scanned by the beam B. Consequently, if at this time, one and the same straight line is printed, there will occur an evil which becomes such as shown in FIG. 5 of the accompanying drawings.
However, it is very difficult to adjust the wavelengths of the laser beams A and B within a range of several nm in the manufacturing process of a laser chip, and it is usual that the relative difference between the wavelengths of A and B becomes different at a probability of 50%. Consequently, when the wavelengths of A and B are selectively used, there has occurred an evil that cost becomes very high.